A radio system utilizing a magnetic field has been conventionally proposed. Unlike radio waves, the magnetic field hardly interacts with human bodies and dielectric materials, and is thus advantageous in forming a definite radio area which is undisrupted by human bodies and obstacles. Moreover, the distance attenuation characteristic of a radio wave is 20 dB/dec., while the distance attenuation characteristic of a magnetic field is 60 dB/dec. Thus, the magnetic field is suitable in the case of definitely defining a radio area boundary.